1. Field of the Invention
The present invention relates to a catalyst which is capable of purifying exhaust gases, such as those produced by motorized vehicles, and in particular, automobiles. More particularly, the present invention relates to a catalyst capable of purifying the exhaust gases of a motorized vehicle which maintains heat resistance and thus good purification efficiency for undesirable and/or harmful gases at high levels over an extended period time.
2. Background of Related Art
As the number of automobiles increases, pollution due to exhaust gases is becoming more and more serious. The chief harmful components of automobile exhaust gases include hydrocarbons, carbon monoxide, and nitrogen oxides (NOx), and the release of these pollutants is more profound in exhaust produced by diesel engines.
At present, the most typical catalyst for purifying the exhaust gases of an automobile is a 3-way catalyst capable of removing carbon monoxide, hydrocarbons and nitrogen oxides at the same time. A typical 3-way catalyst is generally formed by coating a carrier on a honeycomb structure. The carrier is typically impregnated with a noble metal such as platinum (Pt) or rhodium (Rh) and an oxygen absorbing material.
Meanwhile, it is generally beneficial for the carrier employed to have a large surface area in order to increase the contact area between the active metal, which is impregnated in the carrier, and the exhaust gases. .gamma.-alumina is known to satisfy such requirement. That is, a 3-way catalyst employing .gamma.-alumina as a carrier shows a considerably high purification efficiency for harmful 3-way gases, i.e., hydrocarbons, carbon monoxide, and nitrogen oxides (NOx), at ideal conditions in which the air-to-fuel ratio in an engine is 14.7:1.
However, when heated above 1000.degree. C., .gamma.-alumina changes into .alpha.-alumina which has a small specific surface area. Thus, at temperatures above about 1000.degree. C., the reactivity of the catalyst is lowered. Thus, when used for a long time at high temperatures, the activity of the 3-way catalyst rapidly deteriorates, and thus substantial quantities of harmful or undesirable gases are released into the atmosphere.